Focus

Background and Training

Research Summary

Our goal is to understand the interaction between multicellular hosts and their microbial partners, with a focus on symbiotic associations.

The living world is shaped by complex networks of interactions between organisms, some antagonistic and others beneficial. Symbioses allow diverse species to achieve feats unattainable by individual members. These collaborations frequently reflect elegant co- evolution; some are so successful as to have global-scale effects.

We aim to illuminate the molecular basis of highly evolved and intimate forms of symbiosis. Our system is the association between legume plants and nitrogen-fixing bacteria. The two species undergo a complex series of developmental changes, resulting in a specialized symbiotic organ, the root nodule. Within the nodule, bacteria are transformed into intracellular organelles – called symbiosomes – dedicated to converting (fixing) molecular nitrogen into ammonia for the host plant. Because fixed nitrogen is frequently the scarcest soil nutrient for plants, this symbiosis has significant economic and ecological impacts: legume crops provide a large portion of the protein in the human diet without nitrogen fertilizers. Globally, biologically fixed nitrogen constitutes a major component of the nitrogen cycle.

What gives legume species the remarkable ability to provide their own nutrient through an alliance with nitrogen-fixing bacteria? Specifically, what molecular mechanisms allow the host to recognize the appropriate bacteria, internalize the microbes, convert them into specialized organelles, and create the proper cellular environment for nutrient exchange?

Figure 2. In an infected host cell, the endoplasmic reticulum (labeled green by a signal peptidase protein) is in close proximity with newly mature symbiosomes (red).

We use the model legume Medicago truncatula (a relative of alfalfa) to address three research themes:

What is the role of a nodule-specific protein secretory pathway in establishing and maintaining symbiosis?
We identified a protein secretory pathway specifically activated in the nodule to establish symbiosis. Cargo proteins of this secretory pathway unexpectedly include molecules targeted to the intracellular symbiosome compartment. What mechanisms underlie this differential targeting? How is this protein secretory pathway activated and regulated? What is the ensemble of its cargo proteins, and how do they in turn contribute to symbiosis?

Can we identify other necessary metabolic and signaling pathways by cloning additional symbiosis genes?
We are identifying the genes corresponding to M. truncatula mutants unable to sustain productive nitrogen-fixing symbiosis. Identifying these genes should lead to breakthroughs in understanding the molecular mechanisms of symbiosis. For example, one such gene encodes a putative lipase: we will determine whether the protein acts in metabolism, in signaling, or in modulating bacteria internalization.

How can we accelerate the rate of gene discovery, so as to gain insights into nitrogen-fixing symbiosis as a complex system?
A productive nitrogen-fixing symbiosis requires the coordinated action of a large number of host genes. We are developing innovative strategies for high-throughput screens based on gene silencing to gain a comprehensive view of the symbiosis. The methods are applicable to various genetic backgrounds (e.g. in search of suppressors of a mutant phenotype), and thus allow the identification of novel interactions in an emerging biological network of symbiotic genes.

Figure 3. Silencing a PDS gene via a viral vector results in bleached leaves. The method can be applied to silence symbiotic genes.

Publications

Wang D and Dong X. A highway for war and peace: the secretory pathway in plant-microbe interactions. Molecular Plant, 4(4):581 (2011). [PubMed]